The hazy Atlanta skyline as seen from Hartsfield-Jackson Atlanta International Airport in 2006. / Michael A. Schwarz, USA TODAY

by Doyle Rice, USA TODAY

by Doyle Rice, USA TODAY

Scientists, for the first time, have a clearer picture of what causes the "urban heat island" effect, a common phenomenon that makes cities swelter by as much as 22 degrees more than the nearby countryside, especially at night in the summer.

Previously, researchers thought that evaporation was the main reason why rural areas stay cooler than cities. (Evaporation, which absorbs heat energy, is greatly reduced when cement and concrete replace grass, trees and plants.)

Study lead author Lei Zhao of the Yale School of Forestry and Environmental Studies found that convection - how efficiently cities release heat back into the air - is the primary factor in the daytime heat island effect.

The effect is especially true in humid cities such as Atlanta and Nashville, the study found, where this inefficient convection among cities contributes to at least a 5-degree rise in average daytime temperatures.

"There is a synergistic relationship between climate conditions and the urban heat island," said Xuhui Lee, another Yale researcher and study author. "This relationship suggests that the urban heat island will exacerbate heat wave stress on human health in wet climates where temperature effects are already compounded by high humidity."

The study was published Wednesday in the British journal Nature.

In drier climates, such as in the Southwest, the impact is different, and during the day, cities can actually be a bit cooler than the surrounding desert.

At night, the urban heat island effect occurs, no matter how humid or dry the climate is. That's because it's the release of heat stored in buildings and streets that drives the effect at night .

To do the study, scientists plugged satellite data from 65 cities in North America into a computer model that determined how different ingredients (such as radiation, convection, evaporation, heat storage and human-generated heat) come together to create urban heat islands.

Urban heat could have significant impacts on human health in cities worldwide as global temperatures continue to rise - and as more and more people move into cities, Lee said.

More than half the world's population - about 3.5 billion people - live in cities, the World Health Organization reports. By 2050, the WHO projects that 70% of the people on Earth will live in urban areas.